Crystal growth in the presence of finely divided polytetrafluoroethylene

ABSTRACT

Metal salts which are crystallizable from a saturated aqueous solution of the salt in the presence of a dispersion of polytetrafluoroethylene (hereinafter referred to as PTFE) resin are grown as geometrically better defined and larger crystals than those grown from an unseeded saturated solution.

BACKGROUND OF THE INVENTION

Crystals are rarely pure because they generally contain small quantitiesof foreign matter which have been built-in or occluded. Gases, liquidsand solids are readily occluded in a growing crystal; dirt, air andmother liquor are the most common occlusions found in commercialcrystals. From the commercial crystallization point of view, the maininterest lies in finding methods to prevent such occlusions fromoccurring. Vapor occlusions are minimized by avoiding vigorous agitationor boiling. The application of ultrasonic radiation to the system isalso used to prevent bubbles or particles from adhering to a growingcrystal face. Most importantly, the crystallizing system is kept cleanto avoid dirt, other debris and particularly organic contaminants frombeing occluded into a crystal.

This invention is oppositely directed; that is, this processdeliberately provides fine PTFE contaminant particles, in the size rangefrom about 0.05 micron to about 0.5 micron in diameter, as seeds tofacilitate the formation and growth of desirable large crystals in whichthe seeds are beneficially occluded.

Crystallization comprises three fundamental steps: supersaturation (orsuper cooling), formation of crystal nuclei, and the subsequent growthof these nuclei into crystals. Supersaturation may be achieved bycooling, evaporation of the solvent, addition of a precipitating agent,as a result of the chemical reaction, etc., and is vital in anyindustrial crystallizing operation. However, supersaturation alone isnot sufficient to cause crystals to grow. Before the growth stage cancommence, there must exist in the system a number of tiny crystalembryos or nuclei. These may be formed spontaneously, inducedartificially, or deliberately added.

One of the best known methods of inducing crystallization is that ofseeding, that is, the deliberate addition of tiny crystals to the supercooled system. The seeds do not necessarily have to consist of thematerial to be crystallized; crystals of some isomorphous substanceswill often work. In some cases, inert crystalline particles can give thedesired effect. Super cooled systems are sometimes seeded accidentallyby atmospheric dust, but such seeding does not produce the results ofseeding with finely divided PTFE.

It is also well-known that the accidental production of nuclei, referredto as `false grain`, should not be permitted in an industrialcrystallizer. This is undesirable, not for the mass of materialprecipitated, which may be relatively small, but for the number of seedswhich are produced. In any case, it is well-known that if controlledgrowth is required, accidental nucleation must be avoided orcounter-acted, and extreme precautions must be taken to get rid of falsegrain where conventional precautions fail. The deliberate introductionof any contaminant particles, particularly organic particles of asynthetic polymeric resin, is contrary to recognized commercialcrystallization practice.

Crystals of a given substance, produced by different methods, may becompletely different in physical appearance, even though they belong tothe same crystallographic system. This variation in external form iscalled a modification of habit and it results from the inhibition ofgrowth in one direction or the enhancement of growth in another. Themost common cause of habit modification is the presence of impurities;these are absorbed on certain crystal faces and stunt the growth inthose directions. Many complex dyes act as habit modifiers for inorganicsalts and their habit-modifying power depends on their anionic orcationic nature. Selected surface active agents also have been founduseful for habit-modification purposes. Surprisingly, in general, theprocess of this invention has no noticeable stunting effect. Quite tothe contrary, crystals are grown by this process which are generallyuniformly well-defined and larger than crystals grown without seedingwith the PTFE particles, irrespective of the particular type ofindustrial crystallizer used. Crystals of particular lattice structureappear to be more susceptible to the beneficial effects of this processthan others, but surprisingly I have found no inorganic ionic saltcrystals which are detrimentally affected by deliberate seeding, in thespecified preferred ranges, with a dispersion of PTFE resin particles.

Again, it is known that various relationships have been proposed fortheoretically predicting the mass deposition rates on crystal faces, andthese relationships have been used to predict geometrically regularcrystal shapes, the size of a crystal after it has remained in acrystallizing medium for a fixed period of time, and the like. Thecrystallization process of the instant invention does not appear to becharacterized by any known mathematical relationship, and in fact, thereappears to be no simple method of predicting with accuracy the effect ofseeding even in crystals of the same lattice structure.

While it is clearly advantageous to produce crystals as large aspossible, their actual size is only of secondary importance; what reallymatters, with relatively large crystals, is the regularity of theproduct. The less regular the crystals, the fewer voids there will bebetween crystals. Moreover, with less regular, relatively smallcrystals, crystalline fines are generally present which given rise toobjectionable dust which makes handling the crystals in bulk a mostunpleasant task. Of course, besides being essentially dust-free,uniform, large crystal masses have many other desirable properties; theycan be filtered and washed more efficiently during processing, they havegood flow characteristics, and they have a pleasing appearance--animportant sales factor, particularly in the sale of big amounts ofammonium nitrate, ammonium sulfate, and nickel sulfate. Thus, there is aneed for a simple, economical method of producing relatively large,well-defined crystals which may be grown from a supersaturated solution,and which are essentially free from dusty crystalline fines.

Again, relatively small crystals are highly susceptible to caking, evenif the crystals are uniform. Thus there is also a need for a processwhich makes it unnecessary to provide the crystals with a coating agentsuch as is popular for many masses of small crystalline particles, forthe purpose of retaining their free-flowing characteristics. It iscommon knowledge, for example, that table salt is coated with a veryfine dust of magnesium carbonate. Icing sugar is coated with atricalcium phosphate or corn flour. Other anti-caking agents that finduse for industrial purposes include chalk, calcium sulfate, kaolin,diatomaceous earth and the like. Addition of anti-caking agents not onlyis an inconvenience and an economic burden, but generally introduces asignificant level of an undesirable contaminant.

The importance of this process and its several benefits and advantagesare unexpectedly due to the seeding of a saturated solution of inorganicsalts and particularly ionic metal salts with an aqueous dispersion ofsub-micron and micron-size particles of PTFE, as will be explainedhereinafter.

SUMMARY OF THE INVENTION

It is therefore a general object of this invention to provide a new andimproved process for growing geometrically well-defined and/or largecrystals of inorganic and organometallic salts which are crystallizablefrom a saturated solution.

It is a more particular object of this invention to provide a new andimproved process for seeding a saturated solution of an inorganic ororganometallic salt with finely divided wettable particles of PTFE,which despite being a synthetic polymeric resin, surprisingly nucleatesthe solution to effect better crystallization than that effected withoutseeding, and yields crystals of the salt, essentially free of fines.

It is a more specific object of this process to seed a saturatedsolution of an ionic inorganic or organometallic salt with a colloidalsuspension of fibrillatable PTFE to effect better defined or largercrystal growth of monoclinic, prismatic, triclinic, and cubic crystals,than that effected without seeding.

It is another specific object of this process to seed an aqueoussolution of an ionic inorganic or organometallic salt with less thanabout 1 percent by weight, based on total solids precipitated, of finelydivided wettable PTFE, to produce crystals having a predominantlymonoclinic, prismatic or triclinic habit, which are characteristicallybetter defined and/or larger than those crystals produced bycrystallization without seeding.

It is still another object of this invention to provide a novel andunique ionic inorganic crystal seeded by a minute quantity of amorphous,solid, finely divided PTFE particles which unexpectedly initiatenon-epitaxial growth, the occurence of which growth thereaftereffectively propagates epitaxial growth in the normal habit of theinorganic crystal.

It is a particular object of this invention to provide inorganic ioniccrystals grown by the initiation of non-epitaxial growth on an amorphouswettable PTFE resin particle, which crystals have approximately doublethe size of similar crystals grown under identical conditions, exceptfor the seeding with PTFE.

A still further object of this invention is to provide a crystallinemass of inorganic crystals in the size range from about 80 U.S. standardmesh to about 1/4-inch mesh, characterized by an absence of dusty fines,wherein a majority of the individual crystals have occluded therein atleast one solid PTFE particle.

These and other objects, features and advantages of this process and thecrystals grown thereby, will become apparent to those skilled in the artfrom the following description of preferred forms thereof and theexamples set forth herein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

According to the process of this invention, an inorganic crystallinesalt may be crystallized from an aqueous solution of the salt in thepresence of finely divided polytetrafluoroethylene resin particles whichare wettable by the solution.

In each embodiment of the instant invention, only water-wettable finelydivided PTFE is used, such as water-wettable fibrillatable PTFE in theform of finely divided, solid particles commercially available as acolloidal aqueous dispersion concentrated to about 60% by weight ofpolymer having particles about 0.05 to about 0.5 microns in size, withaverage diameters of about 0.2 micron. Another type of fibrillatablePTFE, generally referred to as "fine powder", obtained by coagulation ofthe dispersion is less effective as a seeding agent because it is notwettable in aqueous solutions of inorganic salts. Most notable is thefact that finely divided solid forms of other particulatepolyhalocarbons and polyolfefins are ineffective as seeding agents toeffect the growth of inorganic crystals which possess better appearanceor larger dimensions than those grown without seeding, presumablybecause they are not water-wettable.

U.S. Pat. No. 2,559,752 discloses a process for forming an aqueousdispersion of colloidal water-wettable particles of PTFE resin. It ishypothesized that, were water-wettable finely divided forms of othersynthetic polymeric resins available, they would be similarly useful asseeding agents which exhibit analogous characteristics of a colloidalaqueous dispersion of PTFE.

In general, inorganic crystals are grown from a saturated aqueoussolution of the crystals in the presence of a small amount of PTFE resinin the range from about 0.1 to about 2 percent by weight, based on totalsolids precipitated from the solution. An amount of PTFE in excess of 2percent by weight may be added, but it will be apparent that,particularly where relatively large crystals, greater than about 100mesh, are to be grown, there is no economic justification for theaddition of a substantially larger amount of PTFE than is required toeffect the desired superior crystal growth. Seeding with PTFE appearsgenerally to have no significant effect on yield of crystals on a weightbasis, as compared with an unseeded solution.

Since, in most instances, the yield of crystals from a preselectedsolution to be seeded with PTFE is already known, the amount of PTFE tobe added may be estimated, from one batch to another, with someaccuracy.

It is hypothesized that the process of this invention is characterizedby the initiation of crystal growth, nonepitaxially, by one individualmicroscopic or submicroscopic particle of PTFE, or a small agglomerateof plural, proximately disposed particles. This hypothesis as tonon-epitaxial growth is based on the fact that a PTFE polymer particlehas no crystalline lattice and therefore cannot initiate epitaxialgrowth of any crystal. Crystal growth initiated in this non-epitaxialmanner is thereafter propagated epitaxially in a particular habit, orcharacteristic lattice, in which the crystal would normally grow if itwere not seeded with a water-wettable polymer particle.

With the foregoing understanding of the mechanism of the process, it ispreferable to provide an optimum amount of wettable polymer whichtheoretically corresponds numerically to at least about the number ofcrystals grown from solution, assuming it is desirable to have all thecrystals grown in a geometrically better-defined or larger physical formthan they would otherwise acquire. By geometrically well-defined ismeant that individual crystals are essentially free of stunted growthand notably free of irregular growth on any of the crystal faces. Thegeometrically better-defined growth of PTFE seeded crystals isparticularly apparent when compared to the shape of crystals grownwithout seeding. Most unexpectedly the size of crystals grown by seedingwith fibrillatable wettable PTFE particles of the preferred size rangeis generally more uniform and often larger than crystals obtained byseeding with fine isomorphous crystals. It will be expected that asubstantially lesser amount than the optimum amount of PTFE may notyield as desirable a size distribution, though the mass of crystalsgrown may still be, characteristically, essentially free of crystallinefines having a size range less than about 325 mesh.

Crystals grown by the instant process characteristically contain atleast one occluded PTFE particle. Additional occluded PTFE particles maybe present within the crystal or upon a face of the crystal, butirrespective of the location of additional occlusions, there appears tobe no evidence of a stunting of growth because of the occlusions ofPTFE. It will be recognized that occlusions of individual particles ofPTFE are most difficult to see even under high magnification. Thepresence of the PTFE particles in or on crystals grown with PTFEseeding, is evidenced by the formation of a residue of PTFE when thesecrystals are re-dissolved.

The particular manner in which crystallization is effected is not ofcritical importance and any conventional crystallization process willbenefit from this invention. For example, a single-effect evaporator,working at atmospheric pressure, may have added to it an effectiveamount of colloidal PTFE particles which are uniformly distributedtherein. The solution in the evaporator remains clear as it isconcentrated, and, after transfer to a crystallizer, deposits crystalsas the solution cools. In a similar manner, multiple-effect evaporatorsmay also be seeded with PTFE particles, and yield larger and moreattractive crystals than solutions which are not so seeded.

Alternatively, colloidal PTFE particles may be added to the crystallizerbefore crystals begin to be deposited, and the solution slowly agitatedby a rotating arm or propeller mixer. After crystallization, the moltenliquor is run off and the wet crystals recovered for drying.

Where particularly large and well-defined crystals are to be grown, thesolution may be seeded with crystals which have been previously seededwith PTFE particles. Such crystals may contain PTFE particles occludedon or near their faces, in addition to one or more particles occludedwithin the lattice. When a solution is seeded with PTFE preseededcrystals additional growth occurs epitaxially in accordance withwell-recognized patterns of crystal growth. The occluded PTFE particlesin the preseeded crystals appear to have no unusual effects on furthergrowth, but most surprisingly, PTFE particles at or near the surface ofthe crystal faces appear to aid, rather than hinder, crystal growth. Afurther additional quantity of PTFE particles may be used, if desired,in those instances where an additional beneficial effect is obtained bysuch addition, as for example, noticeably faster crystallization.

The crystallization process of this invention is particularly effectivein the growth of ionic inorganic and organometallic salts which arecrystallizable from saturated aqueous solutions, irrespective of thecharacteristic habit in which they are normally formed. In thoseinstances where a salt is crystallizable in more than one habit from anunseeded saturated solution, seeding with PTFE particles appears to haveno noticeable effect on the formation of crystals with different habits,except that the crystals are larger and have better appearance. Thiscrystallization process is most particularly directed to the growth ofmonoclinic, prismatic, triclinic and cubic crystals which may be formedand grown in the presence of less than two percent by weight ofcolloidal PTFE particles, based on total solids precipitated fromsolution, and more preferably in the presence of from about 0.01 toabout 1.0 percent by weight PTFE.

Ionic inorganic salts which may be grown by the process of thisinvention include the salts of metals forming cations from Groups I, II,III, VI, and VIII including the salts of mixed metals thereof forexample, potassium aluminum sulfate, potassium dichromate and the like.

Ionic organometallic salts which may be grown by the process of thisinvention include the metal salts particularly of Groups I or VIII, ofthe lower carboxylic acids having less than six carbon atoms such asnickel and sodium acetate.

The following examples are by way of illustration only, and thisinvention is not limited by the particular inorganic salts andorganometallic salts described hereinbelow.

EXAMPLE 1

A one-liter solution of nickel acetate is prepared by adding 356 gms ofNi(CH₃ COO)₂.sup.. 4H₂ O and 84 gms. glacial acetic acid to 600 ml.water. A temperature of 80°-90°C. is maintained to ensure dissolution ofthe organometallic salt and additional water is added to bring thevolume to one liter. A very small, but noticeable amount of solid wouldnot dissolve and is removed by filtration through a Buchner funnel. Thefiltrate has a specific gravity of 1.145 at 80° C.

Each of two 250 ml. portions of the solution are seeded with 2.66 g. ofTeflon* T-30 aqueous colloidal dispersion (about 1.6 g. of PTFE solids)of PTFE, and 1.6 g. of Teflon* T-6 PTFE fine powder respectively. Athird 250 ml. portion is maintained as a control and contains no PTFE.

Each 250 ml. portion at about 80° C. is contained in a tall 300 ml.beaker provided with a Teflon coated stirring bar. Each portion isstirred slowly and continuously. It is preferred to make the additionsof PTFE slowly while stirring to maintain the polymer particles ashomogeneously dispersed as possible. The T-6 fine powder is not wettableand an attempt to pre-wet the powder with methanol was ineffective toprovide a homogeneous dispersion.

Stirring of the hot solutions is continued overnight, and thetemperature falls to room temperature of about 23° C. A mass of crystalscrystallizes from each of the three portions which are further chilledto about 15° C. in a water bath while stirring is continued for a periodof about two hours. Each portion is filtered through a Buchner funneluntil relatively dry and the mass of crystals obtained from each portionare weighed and thereafter dried in an oven at 100° C. for thirtyminutes while moderate shear is applied to each mass of crystals whileit is drying. The crystals are prismatic.

    ______________________________________                                                        Funnel Oven     % loss                                                        dry wt.                                                                              dry wt.  after                                                         gms.   gms.     drying                                        ______________________________________                                        Control           74.1     59.2     20.1                                      Portion with T-30 *Teflon                                                                       67.3     59.6     11.4                                      Portion with T-6  *Teflon                                                                       83.4     69.4     16.8                                      ______________________________________                                         *Registered Trademark                                                    

A comparison of the appearance of the dried crystals indicates that thecrystals with T-30 are less white and more green than the others. Inaddition the crystals with T-30 flowed more freely and were remarkablyfree of fines thus permitting the mass to be handled without a mask. Bycontrast, the crystals obtained from the control are characteristicallydusty and the crystals with T-6 show only moderate improvement.

Microscopic examination of the crystal masses indicates that thecrystals with T-30 are geometrically better defined and larger than thecrystals of either the control or the crystals grown in the presence ofT-6 fine powder. Control crystals range in size up to 250 microns, themajority being between 40- 150 μ. Crystals from the T-30 treated portionrange in size up to 1300 μ the majority being in the range from 100-400microns.

It will be apparent that the low percentage loss of moisture afterdrying the crystals containing T-30 produces an economic benefit in thatthe dried crystals may be obtained more quickly and therefore at lesscost.

EXAMPLE 2

Potassium dichromate crystals are obtained from a control aqueoussolution as follows:

A. 75 gms. of potassium dichromate are dissolved in 100 ml. water at 90°C. The hot solution is filtered through a Buchner funnel to removeinsoluble impurities. The hot solution is stirred slowly andcontinuously while depositing crystals and cooling overnight to roomtemperature. It is thereafter cooled to 10°C. in an ice bath. Thecrystals are separated from solution by filtering through a Buchnerfunnel until relatively dry and thereafter dried for 5 hours at about75° C. under vacuum.

B. In the same manner as described immediately hereinabove anotherpotassium dichromate solution is prepared and maintained at 90° C. while2.5 gms. of Teflon T-30* dispersion (about 1.5 gms. PTFE) is added andhomogeneously dispersed therein. The solution containing T-30 PTFE ispermitted to deposit and grow crystals while cooling overnight to roomtemperature. It is thereafter chilled in an ice bath at 10° C., thecrystals separated and dried at 75° C. for 5 hours under vacuum.

Microscopic examination of the crystals obtained from the control andthe T-30 treated solution discloses that the latter are approximatelytwice as large. There is no visual indication of the presence of PTFEparticles in or on the crystals, though agglomerates of PTFE areobserved. These agglomerates of PTFE appear to be excess PTFE particleswhich were not occluded by the crystals. The crystals are triclinic.

EXAMPLE 3

In a manner analogous with that described in Example 2 hereinabove,potassium aluminum sulfate crystals of relatively larger size and bettergeometrical definition than unseeded alum crystals, are grown whenseeded with wettable PTFE particles of T-30 dispersion. The crystals aremonoclinic.

EXAMPLE 4

A finely divided non-fibrillatable powder, of Whitcon No. 5 PTFE istreated with a conventional wetting agent to render it wettable. In amanner analogous to that described in Example 2A hereinabove, a nickelacetate solution is prepared and treated with the treated, wettable,non-fibrillatable PTFE in the same amount, that is, 1.5 gm. PTFE in 75gms. of potassium dichromate. Crystals recovered have PTFE particlesoccluded therein and are better defined and larger than crystalsobtained in Example 2A.

Modifications, changes, and improvements to the preferred forms of theinvention herein disclosed, described, and illustrated occur to thoseskilled in the art who come to understand the principles and preceptsthereof. Accordingly, the scope of the patent to be issued hereon shouldnot be limited to the particular embodiments of the invention set forthherein, but rather should be limited by the advance by which theinvention has promoted the art.

What is claimed is:
 1. A crystal of an inorganic ionic metal saltcrystallizable from an aqueous solution of said salt, said crystalconsisting essentially of said inorganic ionic metal salt and at leastone particle of water-wettable polytetrafluoroethylene polymer in a sizerange from about 0.05 μ to about 0.5 μ, formed in an aqueous suspensoid,said particle being occluded in said crystal which is formed in a sizerange from about 80 U.S. Standard mesh to about 1/4 inch mesh.
 2. Thecrystal of claim 1 wherein said inorganic ionic metal salt has a crystalhabit selected from the group consisting of monoclinic, prismatic andtriclinic.
 3. The crystal of claim 1 wherein said inorganic ionic metalsalt includes a cation of a metal selected from the group consisting ofmetals of Groups I, II, III, VI and VIII of the Periodic Table.